The invention relates to a device for the three-dimensional focusing of an ultrasonic beam.
The invention can be used particularly attractively in the field of medical echography, notably for the formation of images of organs.
Like other medical imaging methods, the trend in echography is towards three-dimensional images. However, this three-dimensional approach imposes problems which are specific of echography. First of all, the information acquisition rate is limited by the propagation speed of the waves in the biological tissues; for example, a speed of 1500 m/s produces 5000 lines/s for a depth of 15 cm. For realizing a plane image with 100 lines and a three-dimensional image with 10 planes, only 5 images will be available per plane and per second; this number is very small and does not enable real-time processing. On the other hand, the observation of organs through acoustic windows of limited dimensions (for example, a passage of 2 cm.times.2 cm between the ribs for the heart) necessitates the use of probes of small dimensions which are not very suitable for three-dimensional imaging where several planes are to be superposed.
At present, three-dimensional images are formed by mechanically displacing a linear array or a single piezoelectric element in order to scan the space to be examined in all directions. However, these known devices have the drawback that they are slow, i.e. the scanning speed is limited by the mechanical movements. This excludes the possibility of parallel acquisition of echographic signals enabling an increase of the rate at which images are formed as proposed in European Patent Application No. 0 210 624.
In an absolute sense, the most obvious solution to the three-dimensional echographic imaging problem would be the use of a probe consisting of elementary piezoelectric transducers arranged in a two-dimensional network. Actually, in association with appropriate electronic circuitry for the formation of channels, this type of probe in theory enables the focusing of the echographic signals in all spatial directions. To this end, it suffices to apply the appropriate phase law to the piezoelectric elements. Moreover, devices for the formation of parallel channels (emission and/or reception simultaneously in several directions) can be used so that the information acquisition rate is increased, resulting in a quasi real-time study of the biological medium. Finally, the problem imposed by limited acoustic windows is solved by angulation of the ultrasonic beam by means of electronic means.
However, even if the described technique at least theoretically solves the specific problems of three-dimensional echographic imaging, it still remains extremely difficult and expensive to carry out such a technique. Actually, the two-dimensional phase network required for obtaining a satisfactory image quality should comprise 64.times.64 elementary piezoelectric transducers with a pitch of approximately .lambda./2. Such a number of transducers (4096) imposes serious problems not only as regards the manufacture of the probe, but also as regards the realization of the electronic circuitry for the formation of channels, which circuitry should amplify, delay and sum 4096 channels.
Thus, the technical problem to be solved by the present invention is to propose a device for the three-dimensional focusing of an ultrasonic beam which comprises a smaller number of elementary piezoelectric transducers so that it is easier to realize and whose electronic control circuitry is simpler and less intricate.